Now Is The Time To Eliminate Bagworms

Article and photos by Claire Stuart

If you have ornamental evergreens, you might be familiar with evergreen bagworms (Thyridopteryx ephemeraeformis). Their little bags dangle from the tips of small branches, resembling bits of dried vegetation or even tiny cones. Bagworms are larvae of moths and are among the commonest pests of evergreens, including pines, spruces, and especially juniper, cedar, and arbor vitae. They occasionally feed on broad-leafed trees.

Bagworms shouldn’t be confused with webworms or tent caterpillars, two entirely different types of caterpillars, both of which live in large groups in communal silk webs that cover branches. Bagworms are also caterpillars, but they live as individuals in well-camouflaged bags made of bits of plant material stuck together with silk.

If you have been curious enough to try to pick one of the little bags apart, you no doubt discovered how tough they are.  And if it was during the growing season, chances are that you found nothing inside. Any hanging bags would have been last year’s. That is because bagworms don’t attach their bags to the plant until the end of their life cycle. The rest of the time, they are walking around feeding in the foliage, and their bags are covered with fresh green material. 

The bagworm wears its bag like a shell and can withdraw into it. It keeps its head and legs outside the bag to eat and walk around, and pulls them in when disturbed. Unlike a snail or turtle shell, the bagworm’s bag is not attached to its body. The bagworm enlarges its bag as it grows, adding 
new vegetation. 

From spring through summer, the caterpillars wander around the trees to eat and grow. When they are mature and ready to pupate in fall, they move out to the tips of branches. They fasten their bags firmly with strong silk, where they hang like tiny Christmas ornaments. Mature caterpillars usually stay on their home tree to pupate, but some wander off, and you might find their bags stuck to the sides of buildings and fences. They seal themselves inside the bags and transform to the pupal stage, becoming adults in fall.

The life of the evergreen bagworm is quite unusual because the male and female moths look like completely different insects. Males have clear wings and look like small flies. Full-grown female moths are wingless and legless and look like slugs. 

The female moths never eat or leave their bags, and their sole function is to mate and lay eggs. They send out pheromone odors to attract flying males, who mate with them in their bags. Males only live a few days. A mated female could live several weeks and will lay about 1,000 eggs inside her bag, then drop to the ground and die.

The eggs overwinter and hatch the next spring. The tiny caterpillars leave the bag immediately, and spin out long strings of silk. They sometimes just drop down to a lower branch, or they might be picked up by the wind and blown away. If they are lucky, they might land on another food plant. 

Once settled, the young caterpillars start making their own bags. They can’t move to another tree unless the plants are touching or close enough together for the caterpillars to crawl there.  Since females can’t fly off to lay eggs elsewhere, large populations of bagworms can build up on a single tree or shrub over several years and can defoliate it. 

The bagworm’s case is extremely tough and is excellent protection against both predators and pesticide sprays. However the strange life cycle of bagworms actually makes it easy to get rid of them if you catch them before a big infestation can build up.

During the growing season, bagworms can be found anywhere on the plant as they feed. They are small and well hidden by the foliage.  By the time they are ready to pupate, they are about an inch long, and they hang their bags from twigs at the outside of the tree. This will allow the caterpillars that hatch in spring to spin out their silk and catch a breeze 
in spring.From winter through early spring, any dangling bags you see will either be empty or will contain overwintering eggs. Pupation will have been completed and adults have emerged. The males have left their bags, mated and died. Females have died and left their eggs in the bags. This is an excellent time to simply remove the hanging bags to get rid of eggs and the potential for infestation next year.  The silk that attaches the bags is very strong, so if you can’t simply pick them off by hand, you should be prepared to use clippers.